[Bug c++/19188] New: friend funtion inside a template class seems to have a problem
A template class(tstack class) has a singly-linked list "struct" declaration in
private section.
Another template class(tstackIterator) - iterator class is declared and defined
in global space.
If the iterator template class trys to get access to the linked list structure
inside the previous class with a friend statement declared in the previous
class, it does not work. It behaves in much the same way as without friend
declaration. The following code shows this:
//: TSTACK.H -- Stack using templates
#ifndef TSTACK_H_
#define TSTACK_H_
// Some implementations require this:
template class tstackIterator;
template
class tstack {
struct link {
T* data;
link* next;
link(T* Data, link* Next){
data = Data;
next = Next;
}
} * head;
int owns;
public:
tstack(int Owns = 1) : head(0), owns(Owns) {}
~tstack();
void push(T* Data) {
head = new link(Data,head);
}
T* peek() const { return head->data; }
T* pop();
int Owns() const { return owns; }
void Owns(int newownership) {
owns = newownership;
}
friend class tstackIterator;
};
template
T* tstack::pop() {
if(head == 0) return 0;
T* result = head->data;
link* oldHead = head;
head = head->next;
delete oldHead;
return result;
}
template
tstack::~tstack() {
link* cursor = head;
while(head) {
cursor = cursor->next;
// Conditional cleanup of data:
if(owns) delete head->data;
delete head;
head = cursor;
}
}
template
class tstackIterator {
tstack::link* p; // this is line 68
// when it is compiled, g++ 3.4.1 give some error message but it does not make
//sense in terms of languae. but the version g++ (GCC) 3.2.3 20030502 works
fine
// except that it gives a warning saying "tstack.h:68: warning: `typename
//tstack::link' is implicitly a typename"
//The Error message is attached to the bottom
///
public:
tstackIterator(const tstack& tl)
: p(tl.head) {}
tstackIterator(const tstackIterator& tl)
: p(tl.p) {}
// operator++ returns boolean indicating end:
int operator++() {
if(p->next)
p = p->next;
else p = 0; // Indicates end of list
return int(p);
}
int operator++(int) { return operator++(); }
// Smart pointer:
T* operator->() const {
if(!p) return 0;
return p->data;
}
T* current() const {
if(!p) return 0;
return p->data;
}
// int conversion for conditional test:
operator int() const { return p ? 1 : 0; }
};
#endif // TSTACK_H_
the compiler complaints are as follows:
In file included from tstktst.cpp:2:
../14/tstack.h:68: error: expected `;' before '*' token
../14/tstack.h: In constructor `tstackIterator::tstackIterator(const
tstack&)':
../14/tstack.h:71: error: class `tstackIterator' does not have any field
named `p'
../14/tstack.h: In copy constructor `tstackIterator::tstackIterator(const
tstackIterator&)':
../14/tstack.h:73: error: class `tstackIterator' does not have any field
named `p'
../14/tstack.h: In member function `int tstackIterator::operator++()':
../14/tstack.h:76: error: `p' undeclared (first use this function)
../14/tstack.h:76: error: (Each undeclared identifier is reported only once for
each function it appears in.)
../14/tstack.h: In member function `T* tstackIterator::operator->() const':
../14/tstack.h:84: error: `p' undeclared (first use this function)
../14/tstack.h: In member function `T* tstackIterator::current() const':
../14/tstack.h:88: error: `p' undeclared (first use this function)
../14/tstack.h: In member function `tstackIterator::operator int() const':
../14/tstack.h:92: error: `p' undeclared (first use this function)
--
Summary: friend funtion inside a template class seems to have a
problem
Product: gcc
Version: 3.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: max656 at hotmail dot com
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188
[Bug c++/19188] friend funtion inside a template class seems to have a problem
--- Additional Comments From max656 at hotmail dot com 2004-12-30 07:41 --- Thanks for the reply. In this one line of code tstack::link* p; "tstack::link " is a typename. so I can not give another name. Actually, I got other ways to get around this problem. But I am interested in this style as well. The code I am experimenting came from Bruce Eckel's old book. The 2nd edtion uses a different way from this. Anyway the definition of "tstack::link" is not created inside the class. it is used only to declare a private pointer variable named "head".Actually it is NOT defined. I think that's why g++ (GCC) 3.2.3 20030502 give the warning of "implicite typename". But a new version, g++ 3.4.1, checks it out and send out a error message of "../14/tstack.h:68: error: expected `;' before '*' token". I don't have any idea of what this means. It does not make any sense at all. Would you take a look at it and reply to me ? I really appreciate it in advance. Tyson -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188
[Bug c++/19188] friend funtion inside a template class seems to have a problem
--- Additional Comments From max656 at hotmail dot com 2005-01-01 01:37
---
Subject: RE: friend funtion inside a template class seems to have a probl
Hi lerdsuwa
You came up with an interesting topic - class template specializtion.
I didn't know aboou it. I need further research on that.
As you mentioned, the keyword "typename' is added and compiled and then it
works fine.
Thanks a lot for your detailed explanation.
It haunted me a lot. I was completely stuck for a while even though I found
some other ways.
Tyson
>From: "lerdsuwa at gcc dot gnu dot org" <[EMAIL PROTECTED]>
>Reply-To: [EMAIL PROTECTED]
>To: [EMAIL PROTECTED]
>Subject: [Bug c++/19188] friend funtion inside a template class seems to
>have a problem
>Date: 31 Dec 2004 13:23:03 -
>
>--- Additional Comments From lerdsuwa at gcc dot gnu dot org
>2004-12-31 13:23 ---
>The 'typename' keyword is required because later C++ introduces a lot more
>features. Those that interfere with your code are partial specialization
>and specialization. For example, you can now have specialization
>
> template <> class tstack {
> int link;
> ...
> };
>
>Then when you declare
>
> tstack t;
>
>it will use the above declaration, where 'link' is now a member data
>instead of a nested class. So the code
>
> tstack::link* p;
>
>which could be interpreted as declaring a variable which is a pointer to
>type
>'tstack::link' if 'T' is 'int'. But when 'T' is 'bool' it could means
>multiplying a member data with another variable named 'p'.
>
>To resolve the ambiguity, later C++ requires the 'typename' keyword to
>treat as pointer declaration, otherwise it will be treated as
>muliplication.
>Older C++ books don't have this 'typename' keyword but newer good books
>from respected authors coming out in the last few years should.
>(Many new C++ books are still wrong).
>
>I know the parser error message could be improved, and there are plenty of
>bug reports elsewhere about it.
>
>--
>
>
>http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188
>
>--- You are receiving this mail because: ---
>You reported the bug, or are watching the reporter.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19188
